1. Field of the Invention
The present invention relates to a spectral sensitivity composing system in a colorimeter or the like.
2. Description of the Related Art
In a colorimeter for measuring the color values of incident light, the light need to be measured with the spectral sensitivity approximating a predetermined spectral sensitivity, specifically, the standard observer's color matching function recommended by CIE (International Commission on Illumination). With recent increase in the use of monochromatic light sources and light sources composed of a combination of monochromatic lights, such as liquid crystal displays, plasma displays, and LEDs, demand is increasing for the precision in the approximation.
In order to realize a predetermined spectral sensitivity, the following two methods have been used conveniently. In a first method, a plurality of optical filters are used in combination (a filter method, hereafter). In a second method, light to be measured is separated into a plurality of wavelength components by wavelength separating means, and then received respectively by plural elements of light receiving means (pixel sensors, hereafter). The output of each pixel sensor is multiplied each by the specific weight factor, and then added together (spectrophotometric method, hereafter). The difference between the filter and the spectrophotometric method is summarized in the following Table 1.
TABLE 1Pre-NoiseCircuitDynamicMethodProcessingcisionS/NFactorScaleRangeFilterParallel−+++Spectro-Parallel+−Low Signal−+photom-LeveletricSerial+−−Low Signal+−LevelSwitchingNoise
As shown in Table 1, in the filter method where a predetermined spectral sensitivity is approximated by a combination of optical filters, the light to be measured is received by a single light receiving element via the combined filters. Accordingly, the wavelength range of received light is wide and the light receiving element of large receiving area can be used. As the result, this method has an advantage of high signal current resulting good S/N ratio. Additionally filter method has advantages of the small circuit scale and the short processing time as all wavelength components in the spectral sensitivity are received simultaneously, and an advantage of the wide dynamic range. However, restrictions in the available filters and variation in characteristics of each filter cause a difficulty in achieving a desired precision in the approximation. Thus, in recent years, the abovementioned spectrophotometeric method becomes dominant in practice.
In the above-mentioned spectrophotometeric method, the light to be measured in the visible wavelength range of 400-700 nm is separated into 31 wavelength components with 10 nm interval for example by wavelength separator as shown in FIG. 13. Then, each separated wavelength component is received by a corresponding pixel sensor. Each of data representing the wavelength component obtained by processing and AID converting the output signal from each pixel sensor is numerically multiplied by the specific weight factor corresponding to a predetermined spectral sensitivity, and then added together for approximating the predetermined spectral sensitivity such as color matching functions x(λ), y(λ), z(λ) of 2 degree (2°) observer defined by CIE. Thus, the spectrophotometric method has the advantage of a high precision in the approximation. However, as each pixel sensor receives one of 31 wavelength components of narrow wavelength range, problems due to the low signal current from each pixel sensor and the low S/N ratio is unavoidable. Either of parallel processing or serial processing is used for the signal processing in the spectrophotometric method in general. Parallel processing where signals from all pixels are processed in parallel requires as many signal processing circuits as wavelength components causing a significant increase of circuit scale. Serial processing using CCD or other charge transfer devices are used as the light receiving element where signals from the pixels are processed serially and the circuit scale is reduced. However, switching noise associated with the charge transfer degrades the S/N ratio and the narrow dynamic range due to the structure is problematic.
An object of this invention is to provide a spectral sensitivity composing system which precisely approximates a desired spectral sensitivity but is small in circuit scale and high in S/N ratio.